There are probably few, if any, missionaries for The Church of Jesus Christ of Latter-day Saints who haven’t encountered the argument, usually advanced by evangelical Protestants, that the Book of Mormon and the other revelations given through Joseph Smith are illegitimate because the Bible forbids post-biblical scripture.
The proof-text upon which this argument is based occurs at the very end of the Bible, in Revelation 22:18-19:
“For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.”